The optical isolator is an optical device which prevents the transmission of light exiting from an optical component or system (e.g., a laser system or an amplifier) back into that optical component or system. That is, an optical isolator prevents back-reflected light from re-entering the optical component or system. Usually, optical isolators allow transmission of light in a single direction and prevent transmission of light in the opposite direction. Back reflected light, which re-enters an optical system might be amplified in an active gain fiber within the optical system and reach high peak power levels in an unregulated manner. The back reflected light can cause damage to any component of the optical system it encounters along the way.
Amplifier chains may contain several isolators between different amplification stages. Thus, optical isolators are further employed for suppressing and stabilizing amplified spontaneous emission, thereby mitigating, at least to some extent, the detrimental effects thereof (e.g., loss of signal peak power and stochastic spiking).
Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematic illustration of a Faraday optical isolator, generally referenced 10, constructed and operative as known in the art. FIG. 1B is a schematic illustration of a polarization independent Faraday optical isolator, generally referenced 50, constructed and operative as known in the art. Reference is now made to FIG. 1A, which includes a first polarizer 12 (i.e., input polarizer), a faraday rotator 14 and a second polarizer 16 (i.e., output polarizer or analyzer). Faraday rotator 14 is optically coupled between first polarizer 12 and second polarizer 16. First polarizer 12 is polarized vertically. Second polarizer 16 is polarized at 45 degrees.
Faraday rotator 14 is an optical component which rotates the polarization angle of light passing therethrough due to the Faraday Effect. It is noted that the terms “polarization angle” and “polarization direction” are used interchangeably herein below. Faraday rotator 14 is a magneto-optic device, in which light is transmitted through a transparent medium (not shown) and is exposed to a magnetic field. The magnetic field lines are substantially parallel to the direction of the beam of light. The polarization direction of the light is continuously rotated during the passage through the medium of the Faraday rotator 14. The total rotation angle β is calculated by the following equation:β=VBL,  (1)where V is the Verdet constant of the material, B is the magnetic flux density (in the direction of propagation of the light beam), and L is the length of the rotator medium. It is noted that the Verdet constant exhibits frequency dependence, such that it decreases with the frequency.
Faraday rotator 14 is a non-reciprocal optical propagating component). For example, Faraday rotator 14 rotates the polarization of a light beam passing there-through in a first direction by an angle β. Suppose that light beam is back-reflected, Faraday rotator 14 does not rotate the polarization of the light beam by an angle of −β. Instead, Faraday rotator 14 further rotates the polarization of the back-reflected light beam by the angle β. Thus, the polarization of the back reflected light beam is rotated by an angle of 2β after passing twice through faraday rotator 14 (i.e., passing once in each direction). In the example set forth in FIG. 1A, Faraday rotator 14 rotates the polarization of light by an angle of 45 degrees. In this manner, the polarization of light passing twice there-through is rotated by ninety degrees.
Optical isolator 10 is a polarization dependent isolator. Polarized beam of light 18A is vertically polarized as shown by corresponding polarization state 24A. Beam 18A passes through first polarizer 12 and remains vertically polarized. In case some portions of beam 18A are not vertically polarized these portions are deflected by first polarizer 12 and do not pass there-through.
Polarization state 24B represents the vertical polarization of beam 18B. Beam 18B enters Faraday rotator 14. Faraday rotator 14 rotates the polarization direction of beam 18B by 45 degrees. Polarization state 24C represents the polarization angle of beam 18C. Beam 18C enters second polarizer 16. The polarization angle of beam 18C corresponds to the polarization angle of second polarizer 16 and therefore beam 18C passes through second polarizer 16 with substantially no losses. Beam 18D exits second polarizer 16. The polarization angle of beam 18D is substantially similar to that of beam 18C, and is represented by polarization state 24D.
At least a portion of beam 18D is back-reflected into isolator 10 as beam 20A. The polarization angle of beam 20A is substantially similar to that of beam 18D. Alternatively, the polarization direction of beam 20A is arbitrary. Beam 20A enters second polarizer 16. Second polarizer 20A filters out any portion of beam 20A which has a polarization direction orthogonal to the polarization direction of second polarizer 16 and of beam 18D (i.e., 45 degrees). Beam 20B exits second polarizer 16 and enters Faraday rotator 14. The polarization angle of beam 20B is substantially similar to that of beam 20A. Faraday rotator 14 rotates the polarization angle of beam 20C by 45 degrees. In this manner, the polarization angle of the entering beam 18A is rotated by ninety degrees after passing twice through Faraday rotator 14. The polarization angle of beam 20C is represented by polarization state 22C. Beam 20C enters first polarizer 12. As the polarization angle of beam 20C is perpendicular to that of first polarizer 12, first polarizer deflects beam 20C. Deflected beam 20D exits first polarizer 12 away from the direction of arrival of beam 18A. In this manner, isolator 10 prevents beam 18A from being back-reflected there-through.
Reference is now made to FIG. 1B, which includes a first polarizer 52 (i.e., input polarizer), a Faraday rotator 54 and a second polarizer 56 (i.e., output polarizer or analyzer). Faraday rotator 56 is optically coupled between first polarizer 52 and second polarizer 56. Each of first and second polarizers 52 and 56, is a birefringent wedge. Faraday rotator 54 is substantially similar to Faraday rotator 14 of FIG. 1A, having a polarization rotation angle of 45 degrees. Optical isolator 50 is a polarization independent isolator, which isolates the optical system from back reflected light regardless of the polarization state of the back-reflected light. That is isolator 50 prevents light, of any polarization state or combination of polarization states, from being back-reflected there-through.
Arbitrary polarized beam of light 58, having arbitrary polarization direction, enters first polarizer 52 at a point of entrance 66. First polarizer 52 separates beam 58 into two orthogonally polarized beams 60A and 60B. Each of beams 60A and 60B enters Faraday rotator 54 and the polarization angle thereof is rotated by 45 degrees. Each of beams 60A and 60B enters second polarizer 56. Second polarizer 56 re-combines beams 60A and 60B back into arbitrary polarized beam 58.
Beam 58 is back-reflected toward isolator 50 as arbitrary polarized beam 62. Beam 62 enters second polarizer 56. Second polarizer 56 separates beam 62 into two orthogonally polarized beams 64A and 64B. Each of beams 64A and 64B enters Faraday rotator 54. Faraday rotator 54 rotates the polarization angle of each of orthogonally polarized beams 64A and 64B by an angle of 45 degrees. Each of beams 64A and 64B enters first polarizer 52. First polarizer 52 diverges beams 64A and 64B, such that each of beams 64A and 64B is back-reflected through isolator 50 with an off-set with respect to point of entrance 66 of beam 58. In this manner, each of beams 64A and 64B is prevented from being back-reflected through the optical path of beam 58.
Ideally the sub-components of a faraday isolator (e.g., beam splitter, faraday rotator) are transparent. However, in practice the sub-components will absorb a small portion of the light passing there-through. This light absorption is termed “parasitic absorption” and is manifested as heating of the sub-components which absorb light.
Optical isolators operating at very high optical power levels, over few hundreds of miliwatts, need to address several issues. For high average optical power levels, parasitic absorption in the Faraday rotator might cause substantial thermal lensing, which distorts the light beam and can also affect the degree of isolation of the optical isolator.
Additionally at very high optical power levels, the Faraday medium of the Faraday rotator should exhibit a high transparency in the spectral region of the passing light beam, a high optical quality (e.g., the uniformity of the material and the parallelism and smoothness of the surfaces), high optical damage threshold and high Verdet constant. As detailed herein above, the rotation of the polarization angle within the Faraday rotator depends on the Verdet constant of the medium, the magnetic flux density, and the length of the rotator medium. For sufficiently rotating the polarization of the passing light beam (i.e., at an angle of 45 degrees), the Faraday rotator requires high magnetic field and long enough medium. The high magnetic field is produced by cumbersome and heavy magnets. The crystals employed as Faraday rotator medium should be long enough (e.g., few centimeters) and have an aperture which is large enough to ensure high optical damage threshold (e.g., a few millimeters), and are therefore expensive.
Raman scattering is inelastic scattering of incident photons which interact with the molecules within a Raman medium. Scattered photons either receive energy from (i.e., anti-Stokes shifted) or lose (i.e., Stokes shifted) energy to the vibrational modes of the optical medium. In this manner, the frequency of the scattered photons is shifted. The frequency shift is related to changes in the vibrational and rotational properties of the molecules of the medium through which the scattering occurs. That is, the energy lost or gained by the scattered photon is gained or lost, respectively, by the scattering molecule in form of vibrational or rotational energy. The scattered Raman photons (i.e., the frequency of which is shifted) can further undergo a second order Raman shift. For example, a first order Raman scattered photon, which energy is decreased with respect to the energy of the original light beam is in-elastically scattered again and its energy is further reduced.
A Band Pass Filter (BPF) is a device that enables passage therethrough to light having a frequency within a certain range of frequencies. The BPF rejects (i.e., absorbs or reflects) light of frequencies outside of that range of frequencies. The range of frequencies passed by the BPF is referred to as the spectral linewidth (i.e., linewidth or bandwidth) of the BPF. The central frequency within the linewidth, at which the attenuation is minimal, is referred to as the transmission line of the BPF.
A Fiber Bragg Grating (FBG) is an optical component that reflects particular wavelengths of light and transmits all others (i.e., a band-block filter). The FBG is constructed of an optical fiber having a periodic variation to the refractive index of the fiber core. The FBG is constructed, for example, from a germanium-doped silica fiber, which is photosensitive, in that the refractive index of the core changes with exposure to UV light. The germanium-doped silica fiber is exposed to UV light for producing periodic variations in the refractive index thereof across fiber.
US Patent Application Publication No. 2010/0045977 to Puzey, Kenneth A., entitled “Methods of Analyzing Samples Using Broadband Laser Light” describes a laser diode with an optical isolator coupled with the output end thereof. The optical isolator includes an acousto-optic cell, which imparts a frequency shift to the light, equal to the frequency applied to the acousto-optic cell. Light reflected back through acousto-optic the cell receives a second additional frequency shift, which is sufficient to prevent undesirable interaction with the laser diode.